Many computer systems include a fan which moves air over the system's components in order to prevent excessive heating of the components. Many such fans are single speed fans that operate constantly while the computer is in operation.
Some more modern systems utilize variable speed cooling fans. The speed of such fans is increased when more cooling is desired. One such prior art system has a configuration such as that shown in FIG. 1. The system shown in FIG. 1 includes a system controller 1, a fan 2, and a fan controller 3. The connections between the system controller 1 and the fan controller 3 include a power line 1a, a ground line 1b, a Pulse Width Modulated (PWM) signal line 1c and a tachometer signal line 1d. A transistor switch 4 controls the voltage on tachometer line 1d. 
The fan controller 3 is physically attached to the fan motor (not shown in FIG. 1) and it drives the coils in the fan motor causing the fan 2 to rotate. Both the system controller 1 and the fan controller 3 include microprocessors that perform various calculations digitally.
The system controller 1 provides a PWM signal on line 1c which indicates the desired speed of the fan. That is, the duty cycle of the PWM pulses on line 1c specify the desired speed of the fan. The PWM pulses on line 1c are low pass filtered to generate an analog voltage. This analog voltage is passed to an analog to digital converter to generate a digital signal. This digital signal indicates the desired fan speed to the fan controller 3 and this digital signal goes to the microprocessor in the fan controller 3.
The system includes a feedback loop via line 1d. A tachometer in fan controller 3 controls transistor 4. Transistor 4 produces pulses on line 1d that indicate the actual speed of the fan. At the system controller 1, the frequency of the pulses on line 1d is converted to a digital signal and provided to the microprocessor in the system controller.
It is noted that with the system shown in FIG. 1, only one parameter is communicated in each direction. Desired speed is communicated from system controller 1 to fan controller 3 on line 1c. Measured fan speed is communicated from fan controller 3 to system controller 1 on line 1d. 
It is also noted that two signal conversions are required in the system described above. The PWM signal is converted to a digital signal in the fan controller 3 and the frequency signal on line 1d is converted into a digital signal indicating fan speed at the system controller 1. Such conversions are inherently somewhat inaccurate, especially in low cost devices.